Magnetic recording media in the form of tapes or disks have widely been used for data storage. Magnetic heads are commonly employed to perform the tasks of interacting with these recording media.
FIG. 1 shows a conventional magnetic head 2 comprising a flat inductive coil 4 sandwiched between a first yoke layer 6 and a second yoke layer 8. The two magnetic yoke layers 6 and 8 contact each other at a back closure region 10 at one end to form a magnetic path 9 and define a narrow transducing gap 12 at another end. During data writing, electrical current representing information passes through a pair of electrical leads 11 and 13 and through the inductive coil 4 to induce magnetic flux along the magnetic path 9. The induced magnetic flux reaches the narrow gap 12 and magnetizes a moving recording medium (not shown) disposed close by.
During data reading, magnetic flux emanating from a recorded medium (not shown) is intercepted by the narrow gap 12. The intercepted magnetic flux flows along the continuous magnetic path 9 defined by the two yoke layers 6 and 8 and induces electrical current in the inductive coil 4. The induced current in the coil 4, which is directed through the electrical leads 11 and 13, corresponds to the data stored on the recording medium.
As shown in FIG. 1, the inductive coil 4 of the head 2 is geometrically flat in topology. As is known in the art, when current passes though a structure, such as the coil 4, induced magnetic flux is mostly generated at the central region adjacent to the axis 14 of the coil 4. It is the back closure region 10, with its relatively wide physical area and high permeability, that captures the induced magnetic flux for transmission to the gap 12 during data writing. The magnetic flux has to pass through a long magnetic path 9 which is defined by the second yoke layer 8. This arrangement is undesirable in several aspects. First, the long magnetic path 9 contributes substantially to the reluctance of the magnetic head 2 and renders the head 2 less effective in flux transmission. To compensate for the inefficiency, the coil 4 is normally wound with a large number of turns. As a consequence, the inductance of the coil is further increased. A magnetic head with high inductance is sluggish in response to writing current during the data writing mode and incapable of reading media at a high rate during the data reading mode. Furthermore, the long magnetic path with the irregular geometrical topology is the main source of magnetic domain instabilities, which is especially enhanced at the back closure region 10 where a highly unstable domain pattern, commonly called the "spider web" pattern, resides. The constant merging and splitting of the unstable magnetic domains in the yoke layers 6 and 8 during operation significantly produces Barkhausen noise (also called popcorn noise) to the head 2 and accordingly lowers the signal-to-noise ratio (SNR) of the head. To compound the situation further, the coil 4 with the large number of windings is also high in ohmic resistance which is a key contributor to Johnson noise. As a consequence, the SNR is further degraded.
To solve the aforementioned problems, different kinds of magnetic heads have been suggested. FIG. 2 illustrates a prior art magnetic head described in Cohen et al., "Toroidal Head Supports High Data Transfer Rates", Data Storage, February 1997, pp 23-28. FIG. 2 shows a magnetic head 16 that includes a toroidal coil 18 formed of two coil segments 18A and 18B. The first coil segment 18A is connected in series to the second coil segment 18B. Electrical leads 20 and 22 are connected to the first and second coil segments 18A and 18B, respectively. The first coil segment 18A wraps around a first yoke layer 24. In a similar manner, the second coil segment 18B surrounds a second yoke layer 26. The two yoke layers 24 and 26 contact each other at a back closure region 28 at one end, and define a narrow transducing gap 30 at another end. With this arrangement, a continuous magnetic path 36 with the transducing gap 30 is defined by the two yoke layers 24 and 26.
During data writing, writing current I passes through the coil 18 via the electrical leads 20 and 22. Magnetic flux is accordingly induced in the coil 18. In a similar fashion as with the coil 4 shown in FIG. 1, the coil segments 18A and 18B, being spiral structures, generate magnetic flux around the areas adjacent to the coil axes 32 and 34, respectively. The induced flux flows directly through the two yoke layers 24 and 26 without relying on the back closure region 28 for flux collecting. The head 16 is more efficient in controlling flux flow, and consequently has better performance.
Advantageous as it appears, the head 16 still requires the coil 18 to be wound with a large number of coil turns. Therefore, the head 16 has undesirable high inductance.
In Cohen et al., the authors are fully aware of the detrimental effects of the high coil inductance on head performance. In fact, Cohen et al. specifically state that the head inductance L is proportional to the square of the number of coil windings N, while the output signal generated by the head 16 only increases linearly with the number of coil windings N. The prior art head 16 is fabricated with a large number of coil turns N, required to effectively drive the two long yoke layers 24 and 26 which are high in magnetic reluctance. There are two coil segments 18A and 18B sandwiched between the two yoke layers 24 and 26 which exacerbate the curvature of the second yoke layer 26. Consequently a longer second yoke layer 26 is required to define the magnetic path 36. With a longer and more curved magnetic path 36, more coil windings are needed to drive the yoke layers 24 and 26 in order to supply sufficient field strength from the narrow gap layer 30. The overall effect is that the head 16 is burdened with a high inductance.
Data storage products are now built with smaller geometrical sizes and with higher storage capacities. To interact with these storage products having narrow track widths and high areal densities, a magnetic head needs to have low head inductance, thereby providing sufficient agility and responsiveness to the head during normal operation. Also, the head must provide a high SNR such that valid signals are not overshadowed by background noise. Furthermore, the head must be small in physical geometry and thus be compatible with miniaturized air bearing sliders which are designed to accommodate the rapid movements of the actuator arms of the disk drives. All of these features impose stringent requirements in the design and manufacturing of a magnetic head.